1. Field of the Invention
This invention relates to a diesel engine.
2. Description of the Related Art
As compared with a gasoline engine, a diesel engine is superior in a thermal efficiency, and has the advantage of withstanding severe conditions (continuous high-load operation). On the other hand, however, a diesel engine discharges nitrogen oxides NOx and soot (particulate discharged substances). Therefore, an exhaust gas of a diesel engine is not clean, so that a diesel engine is environmentally undesirable. Under the circumstances, the reduction of NOx and the reduction of a discharge rate of soot are being required of a diesel engine.
In a diesel engine, a fuel is injected from an injection port of a fuel injection nozzle into a combustion chamber (compression chamber) compressed by a piston, and fuel spray is thereby formed, the fuel being thereby burnt.
In a regular diesel engine, a fuel is injected from an injection port of a fuel injection nozzle into a combustion chamber (cylinder) during a predetermined injection period in which a piston is in a position in the vicinity of a top dead center (T.D.C.), and combustion energy is thereby generated.
However, since the temperature in the combustion chamber becomes high, nitrogen oxides (NOx) occur in large quantities. Moreover, a portion of an excessively rich gaseous mixture locally occurs in the combustion chamber to cause soot to be formed.
Under the circumstances, attempts to hold down the occurrence of nitrogen oxides have heretofore been made by injecting an aqueous emulsion fuel, which is obtained by mixing a fuel and water with each other in advance, from a fuel injection nozzle into a fuel combustion chamber.
When the aqueous emulsion fuel is injected from the fuel injection nozzle into a combustion chamber, the thermal capacity of the interior of the combustion chamber increases due to the influence of the latent heat of evaporation of the water and a high specific heat thereof. This causes a combustion temperature in the combustion chamber to decrease.
When an aqueous emulsion fuel is injected into a combustion chamber, the fuel and water are dispersed uniformly in the combustion chamber, and the particles of the fuel are microscopically put in the condition in which the particles of the fuel a recovered with inert vapor. As a result, an ignition delay occurs, and the combustion of the fuel starts with the mixing of the fuel and suction air with each other promoted. This enables an increase in the temperature to a high level in a local combustion field to be avoided.
Thus, the combustion temperature in the combustion chamber decreases, and an increase in the temperature to a high level in a local combustion field can be avoided, so that the formation of nitrogen oxides can be held down. Since the occurrence of a portion of an excessively rich gaseous mixture in the combustion chamber can be avoided, the formation of soot can be held down.
It is said that, when an aqueous emulsion fuel is used, the occurrence of nitrogen oxides decreases to ⅕.
However, since the aqueous emulsion fuel has to be injected into a combustion chamber after the fuel is mixed with water in advance, it is difficult to carry out an operation for varying a fuel-water mixing ratio arbitrarily in accordance with the magnitude of a load applied on a diesel engine and temperature conditions in a short period of time. Therefore, the aqueous emulsion fuel is necessarily used at a predetermined mixing ratio of the fuel and water.
When a diesel engine is used as a generator operated with a predetermined load by using an aqueous emulsion fuel having a fuel-water mixing ratio of even a predetermined level, a problem does not arise. When a diesel engine is used for a construction machine in which a load and temperature conditions vary incessantly, the combustion condition is deteriorated when a load is at a certain level in certain temperature conditions.
When a larger quantity of water is mixed with the fuel in accordance with a high engine load with the engine operated at, for example, a rated point, a combustion temperature does not attain a suitable level at a low-load operation of the engine, for example, during a low idling time, and unburnt fuel is discharged in large quantities. Conversely, when a smaller quantity of water is mixed with the fuel in accordance with a low engine load, the combustion temperature becomes high at a high-load operation of the engine, and nitrogen oxides are discharged in large quantities.
In view of this, a method of varying a mixing ratio of a fuel to water arbitrarily in accordance with the magnitude of a load and temperature conditions in a short period of time is being tried as follows.
1) The injection nozzles for a fuel and water are provided separately, and the water and fuel injected from the two injection nozzles are mixed with each other in a combustion chamber.
2) Predetermined quantities of water and fuel are supplied alternately to the injection nozzle, and the water and fuel are injected from the same nozzles alternately into the combustion chamber so as to mix them with each other therein.
In all of these methods 1), 2), the fuel is injected into the portion of the interior of the combustion chamber into which the water was just injected, and, therefore, the fuel and water are rarely dispersed uniformly in the combustion chamber. This causes a local combustion field in which the particles of the fuel are not microscopically covered with inert vapor to be formed, and an increase in the temperature to a high level in this local combustion field cannot be avoided. As a result, the formation of the nitrogen oxides cannot be sufficiently held down.